This invention relates to a multi-element fuel cell system as well as methods for their production.
Fuel cells have potential for any application which is currently met by batteries, with the advantages of higher energy per unit weight and volume than batteries. Existing fuel cells require endplates and clamping straps to prevent fuel leakage and minimising surface contact resistances making them too heavy and bulky for use in man-portable equipment.
U.S. Pat. No. 5,336,570 proposes a fuel cell arrangement in which planar cells are positioned opposite one another and sealed at each edge to form a hydrogen store between them, then rolled up on a separator screen to maintain oxygen flow to each cell. Alternatively, single cells are constructed as a closed, generally tubular shape. However, to increase the available power, multiple cells must be connected in an array so extra weight is added in the electrical connections and in provision of a hydrogen supply to each separate cell or pair of cells.
In accordance with a first aspect of the present invention, a multi-element fuel cell system comprises a substantially cylindrical former; a rechargeable hydrogen fuel source; and a plurality of fuel cell elements; wherein the former comprises a series of interconnecting modules; wherein each former module is perforated to allow passage of fuel to the fuel cell elements; wherein each fuel cell element is positioned radially outwardly of the former; each element being provided with channels, arranged to receive and direct fuel gas, an anode current collector, a gasket, a first diffusion backing layer, a membrane electrode assembly, a second diffusion backing layer and a cathode current collector; wherein the cathode current collector applies even compression to the fuel cell element, such that good electrical contact is maintained within each fuel cell element; wherein the fuel cell elements are electrically connected in series via respective anode and cathode current collectors; wherein the fuel cell elements at each end of the former are capped for connection to equipment; wherein the former and current collectors have substantially the same coefficient of thermal expansion; and wherein the fuel source is coupled to the fuel cell elements.
The present invention provides for a fuel cell system which has sufficient power in a man portable size and weight by virtue of a former composed of a series of interconnected former modules on which are mounted fuel cell elements all with access to the same source of hydrogen, reducing the number of end caps and amount of extraneous fuel supply equipment required for the same power output. This feature allows a fuel cell system to be constructed to any required size using only one fuel source. This is crucial for the design of man portable equipment where power to weight ratio must be maximised and is a feature which has not been addressed in any prior art. An additional design advantage of the invention is one of ease of repair in use. A former module containing a defective fuel cell element can simply be replaced allowing the continued use of the power source. Replacement modules may be supplied with man portable equipment with little weight penalty thereby ensuring equipment effectiveness in remote areas where backup support is limited. Further, the invention uses one of the current collectors to provide compression, such that good electrical contact (i.e. surface resistance minimised) is maintained within each fuel cell element, avoiding the use of an additional non-functional element. In addition, the coefficients of thermal expansion for the current collector and former are chosen to be similar, so that thermal cycling does not cause a reduction in performance because of an increase in interfacial resistance due to a reduction in the level of compression over time.
Examples of former material include Tufnol(trademark) and stainless steel. These materials have very similar coefficients of thermal expansion so that thermal cycling in operation has minimal effect on efficiency. Preferably, the cathode current collector and the former comprise stainless steel. Where the former is made of stainless steel, preferably the former acts as the anode current collector.
Another cause of loss of performance, particularly when the system is operating at high ambient temperatures is dehydration. At higher current densities, cell performance degrades steadily with time. The rise in temperature at higher currents, and electro-osmosis can both contribute to increased rate of loss of water from the polymer electrolyte membrane of the membrane electrode assembly. Since the membrane""s conductivity is dependent on how much absorbed water it contains, the system""s performance will suffer as the membrane dries out. In equilibrium, the membrane needs to be kept fully moist, but not have so much water that the cell is flooded, so the rate of generation and loss of water need to be kept in balance. Water management may be further aided by the use of a suitably treated gas diffusion layer, such as Carbel(trademark) or Carbel CL(trademark), manufactured by W. L. Gore and Associates, USA, which assists by helping to maintain the water content of the MEA at, or about, the optimal level.
Preferably, the system further comprises an outer porous, hydrophobic layer, such that rate of loss of water from the membrane electrode assembly during operation is optimised, without affecting air transport. This layer is preferably chosen from one of perforated cellulose wrapping, man-made fibrous cloth, water-proofed cotton cloth, expanded polystyrene and polyimide foam.
Alternatively or additionally, a fine weave wire mesh may be provided inside the cathode current collector. This will also shield the surface of the assembly from excessive water loss through exposure to the air, and so help maintain or improve performance.
Preferably, the system further comprises an impervious outer shell and means for ensuring air flow to the fuel cell elements beneath the outer shell. The use of an external container helps to optimise the ambient humidity, thus also helping with performance. The air flow may be achieved using a small fan at one end of the tube or providing a separate oxygen supply. The separate oxygen supply is suitable where the ambient air contains contaminants, for example urban pollutants such as benzene or carbon monoxide. The oxygen supply allows the system to be sealed to prevent ingress of pollutants which reduce performance, yet allowing continued operation.
Preferably, the hydrogen fuel source comprises one of a hydrogen store or a hydrogen generator. A hydrogen store may be refilled with hydrogen, whereas hydrogen generators need replacement, but give generally better performance for the same weight.
Preferably, the hydrogen source comprises a metal hydride of up to 2 wt% H2, a primary hydride, compressed hydrogen or hydrogen stored in carbon nanofibres.
Preferably, the rechargeable hydrogen source is provided in a replaceable cartridge. This would make replacement of the source by the user practical.
Any suitable catalyst may be used in the membrane electrode assembly, but preferably the catalyst comprises platinum deposited on a carbon support. To further improve performance, preferably, the catalyst comprises between 0.2 and 1.0 mg of platinum per cm2.
In accordance with a second aspect of the present invention, a method of manufacturing a multi-element fuel cell comprises providing a cathode current collector, a first diffusion backing layer, a membrane electrode assembly, and a second diffusion backing layer to form a stack; folding the stack around a former; applying a predetermined pressure to the cathode current collector to ensure good contact between the stack and the former; fastening the cathode current collector in place to form a fuel cell element; and connecting a plurality of fuel cell elements together to form a fuel cell.
An embodiment of a method of manufacturing a multi-element fuel cell system of the invention, comprises extruding a series of substantially cylindrical former modules wherein the former modules act as the anode current collector; perforating the former modules; applying a first diffusion backing layer, a membrane electrode assembly, a second diffusion backing layer and a cathode current collector in register with the anode current collector to form individual fuel cell elements; fastening the cathode current collector such that it applies compression to the fuel cell element; assembling a series of complete fuel cell elements to produce a cell of the required size and connecting the anode current collector and cathode current collector of adjacent fuel cell elements in series; capping each end and providing connectors for connecting to equipment in use.
The present invention allows fuel cells to be manufactured to different power specifications using the same production process. Furthermore, one or more multi-element fuel cells of the invention may be linked together, in any suitable configuration to create a larger power source. The connection could, for example, be arranged in such a way that although the width of the overall power source may be increased, its length is not affected, if that is desirable in the particular situation in which the fuel cells are to be used.
Preferably, the method further comprises applying a porous, hydrophobic layer over the cathode current collector, such that rate of loss of water from the system is optimised.